Add support for line item options

This adds a foreign key from Transfer to LineItemOptions (as suggested by Jordan in #1119) and updates the accounting controller and StudentExtraCosts module to make use of it.  You can test this by creating new line item types for a program; you can add options (each with a description and optionally a new cost value) from the admin page for the line item type.  You should see each of these options on the extra costs page in student reg, and it should remember your choices when you save and come back later.

Addresses #1119 and #1120.  However, it could be argued that more front-end work is desirable for #1120.  Also, we don't support a combination of multiple LineItemOptions choices and variable quantity (in the StudentExtraCosts module; the accounting controller should be able to handle it).

I think we should leave ourselves open to the possibility of adding checkboxes/quantities for line item types with options.  The accounting controller should be be fine with it (since it folds quantities into those tuples), or am I missing something?

As you say, we would need to fix up StudentExtraCosts.  I don't think that's a bad thing; I believe most of that code was cobbled together on an airplane.